public class Cal {
    public static void main(String[] args) {
        double sum =0;
        for (int i = 2;i <= 100;i += 2) {
            sum+=(1.0/i);
        }
        for (int i = 1;i <= 99;i += 2) {
            sum-=(1.0/i);
        }
        System.out.println(sum);
    }
}